The invention is in the field of electronic communication apparatus. A serious problem in electronic communications is caused by extraneous "noise" signals received with the information signals. Noise signals are generated by various natural and manmade agencies, and their suppression is a design problem of major proportions in the manufacture of communications equipment. Noise signals generated deliberately by hostile agencies for the purpose of "jamming" communication signals are a serious problem. Here powerful noise signals are transmitted in the communication channels in an effort to "jam" or impede communication.
Flexible and reliable communications are becoming an increasingly important factor in warfare. As a result, the impact of enemy jamming on communications facilities is receiving ever increasing attention. One of the simplest jamming techniques is to use a continuous wave (CW) or slowly varying (Swept CW) jammer. This can be implemented with simple oscillators and narrow band power amplifiers. Generally speaking, the anti-jam strategy is to develop sufficiently effective countermeasures. In the case of narrow band (CW) jamming, an effective countermeasure would force the jammer to more "wideband" techniques wherein his effectiveness can be partially mitigated. A corresponding example exists for "pulse" jamming. In this case a jammer can get "peak" power advantage over unprotected communications transmitters and the anti-jam technique is to employ interleaved coding which essentially spreads the effect of the jammer over larger time intervals.
CW jamming can be counteracted in two ways, i.e. (1) by filtering and (2) by cancellation. The cancellation technique involves a two-loop (phase-amplitude) cancellation feedback control system. By comparison filter techniques are not sensitive to either the level or the phase of the jammer but can have a degrading effect on the desired signal. An ideal filtering approach would be to cause a "tracking" very narrow band stop filter to track the CW signal. To the extent that the filter has a non-zero bandwidth, is also the extent to which the desirable communications signal is suppressed. Therefore it is desirable to use a filter with a stop-band as narrow as possible. At intermediate frequencies of 70 or 700 MHz (typical IF frequencies used in military equipment) the state-of-the-art filter design imposes severe restrictions. The invention is an alternative to conventional tracking filter approaches.